This invention relates to a two-stage cascade refrigerating system and particularly relates to the structure of a receiver.
A two-stage cascade refrigerating system conventionally includes a primary side refrigerant circuit and a secondary side refrigerant circuit each of which effects a refrigerating operation individually, as disclosed in Japanese Unexamined Patent Publication No. 9-210515. This two-stage cascade refrigerating system is used for obtaining temperatures as low as minus few ten degrees. This two-stage cascade refrigerating system is advantageous in energy saving since it can be used in an efficient compression ratio from a high compression ratio to a low compression ratio.
The primary side refrigerant circuit of the two-stage cascade refrigerating system is formed by connecting a compressor, a condenser, an expansion valve and an evaporation section of a refrigerant heat exchanger in this order. On the other hand, the secondary side refrigerant circuit is formed by connecting a compressor, a condensation section of the refrigerant heat exchanger, an expansion valve and an evaporator in this order. In the refrigerant heat exchanger, heat of condensation in the secondary side refrigerant circuit is exchanged with heat of evaporation in the primary side refrigerant circuit.
In conventional two-stage cascade refrigerating systems including the above-mentioned two-stage cascade refrigerating system, the evaporator for a secondary refrigerant is frosted and therefore a defrosting operation is carried out, for example, at predetermined time intervals. As an exemplary technique of implementing such a defrosting operation, there has been proposed one which carries out a defrosting operation by changing the directions of refrigerant circulation in the primary and secondary side refrigerant circuits to respective reverse cycles.
Specifically, the primary and secondary side refrigerant circuits are provided with four-way selector valves, respectively. The primary side refrigerant circuit provides refrigerant circulation such that the refrigerant flows through the compressor, the refrigerant heat exchanger, the expansion valve and the condenser in this order and returns to the compressor. On the other hand, the secondary side refrigerant circuit provides refrigerant circulation such that the refrigerant flows through the compressor, the evaporator, the expansion valve and the refrigerant heat exchanger in this order and returns to the compressor. As a result, the frost on the evaporator in the secondary side refrigerant circuit is melted by a high-temperature refrigerant from the compressor.
Further, conventionally, the primary side refrigerant circuit is provided with a receiver between the condenser and the expansion valve to regulate a liquid refrigerant, whereas the secondary side refrigerant circuit is provided with a receiver between the refrigerant heat exchanger and the expansion valve to regulate a liquid refrigerant. However, there has been a problem in such primary and secondary side refrigerant circuits in that they cannot control a liquid refrigerant at a suitable flow rate during the defrosting operation.
Specifically, during the defrosting operation, the condenser in the primary side refrigerant circuit functions as an evaporator while the evaporation section of the refrigerant heat exchanger functions as a condenser. If the outdoor air temperature is high at the time, the evaporation capability of the condenser is increased whereas the condensation capability of the evaporation section of the refrigerant heat exchanger is steady, which causes a so-called wet operation in the system.
That is, in the receiver, two pipes introduced into its container have conventionally been set to be oriented downward. Therefore, if the liquid refrigerant in the receiver is large in amount, a large amount of liquid refrigerant will return to the compressor via the condenser. As a result, the system enters a so-called wet operation, which invites a problem of poor reliability.
Particularly for a long pipe which connects the compressor and the refrigerant heat exchanger through a large distance, the amount of refrigerant charged thereinto is essentially large. Therefore, the liquid refrigerant will be stored in large amounts in the receiver. This makes it impossible to satisfactorily prevent back of the liquid refrigerant into the compressor.
On the other hand, in the secondary side refrigerant circuit during the defrosting operation, the evaporator functions as a condenser while the condensation section of the refrigerant heat exchanger functions as an evaporator. Because of the relationship between the compressor and the evaporator which are disposed in proximity to each other, the amount of refrigerant charged into the secondary side refrigerant circuit is small. In addition, the capacity of the evaporator is large. The liquid refrigerant is therefore difficult to accumulate in the receiver. As a result, the refrigerant is difficult to return to the compressor, which makes it difficult to ensure a desired refrigerant circulating flow rate. Particularly if no pressure reduction capability is provided between the receiver and the refrigerant heat exchanger, the suction side pressure of the compressor easily drops down to a low level, which makes it impossible to ensure the desired refrigerant circulating flow rate.
The present invention has been made in view of the foregoing points and has its object of controlling a liquid refrigerant at a suitable flow rate during a defrosting operation.
Specifically, as shown in FIG. 1, a two-stage cascade refrigerating system as a first solution includes a primary side refrigerant circuit (20) which is formed by connecting a compressor (21), a condenser (22), an expansion mechanism (EV11) and an evaporation section of a refrigerant heat exchanger (11) in this order and in which a primary refrigerant circulates and a receiver (25) is disposed in a liquid line. The system also includes at least one secondary side refrigerant circuit (3A) which is formed by connecting a compressor (31), a condensation section of the refrigerant heat exchanger (11), an expansion mechanism (EV21) and an evaporator (5a) in this order and in which a secondary refrigerant circulates, a receiver (34) is disposed in a liquid line and the primary refrigerant exchanges heat with the secondary refrigerant in the refrigerant heat exchanger (11).
Further, said at least one secondary side refrigerant circuit (3A) and the primary side refrigerant circuit (20) are arranged to make the direction of refrigerant circulation reversible between a forward cycle and a reverse cycle. In addition, the receiver (25) of the primary side refrigerant circuit (20) includes a container (2a), a first pipe (2b) which communicates with the condenser (22) and is introduced to the inside of the container (2a) and an opening end of which is located at an inside upper position of the container (2a), and a second pipe (2c) which communicates with the refrigerant heat exchanger (11) and is introduced to the inside of the container (2a) and an opening end of which is located at an inside lower position of the container (2a).
A second solution is directed to a two-stage cascade refrigerating system including the primary side refrigerant circuit and the secondary side refrigerant circuit like the first solution. Further, said at least one secondary side refrigerant circuit (3A) and the primary side refrigerant circuit (20) are arranged to make the direction of refrigerant circulation reversible between a forward cycle and a reverse cycle.
Furthermore, the receiver (34) of the secondary side refrigerant circuit (3A) reversible in refrigerant circulation includes a container (3a), a first pipe (3b) which communicates with the refrigerant heat exchanger (11) and is introduced to the inside of the container (3a) and an opening end of which is located at an inside lower position of the container (3a), and a second pipe (3c) which communicates with the evaporator (5a) and is introduced to the inside of the container (3a) and an opening end of which is located at an inside lower position of the container (3a).
In addition, a pressure reduction passage (65) for allowing the flow of the secondary refrigerant therethrough during the reverse cycle of refrigerant circulation alone is provided between the refrigerant heat exchanger (11) and the receiver (34) in the secondary side refrigerant circuit (3A) reversible in refrigerant circulation, and the pressure reduction passage (65) is provided with a shut-off valve (SVDL) smaller in diameter than the passage.
A two-stage cascade refrigerating system as a third solution is arranged in the second solution so that, like the first solution, the receiver (25) of the primary side refrigerant circuit (20) includes a container (2a), a first pipe (2b) which communicates with the condenser (22) and is introduced to the inside of the container (2a) and an opening end of which is located at an inside upper position of the container (2a), and a second pipe (2c) which communicates with the refrigerant heat exchanger (11, 11) and is introduced to the inside of the container (2a) and an opening end of which is located at an inside lower position of the container (2a).
A fourth solution is concerned with the first or second solution, wherein a plurality of refrigerant heat exchangers (11, 11) are provided. Further, the evaporation sections of the refrigerant heat exchangers (11, 11) are connected in parallel with each other to form the primary refrigerant circuit (20), and the refrigerant heat exchangers (11, 11) are connected with the secondary side refrigerant circuits (3A, 3B), respectively. Furthermore, at least one secondary side refrigerant circuit (3A) of the plurality of secondary side refrigerant circuits (3A, 3B) is arranged to make refrigerant circulation therein reversible. In addition, the evaporators (5a, 5b) of the secondary side refrigerant circuits (3A, 3B) are formed unitarily.
In these solutions, during a defrosting operation, the primary side refrigerant circuit (20) and the secondary side refrigerant circuit (3A) together provide refrigerant circulation in reverse cycles. Particularly in the fourth solution, one secondary side refrigerant circuit (3A) alone effects a defrosting operation.
On one hand, in the secondary side refrigerant circuit (3A), the shut-off valve (SVDL) of the pressure reduction passage (65) is fully opened. The secondary refrigerant thereby discharged from the compressor (31) flows through the evaporator (50) to heat the evaporator (50) and defrost the evaporator (50). Thereafter, the secondary refrigerant flows through the receiver (34) and the pressure reduction passage (65) and is then reduced in pressure in the shut-off valve (SVDL). Subsequently, the secondary refrigerant evaporates in the condensation section of the refrigerant heat exchanger (11) and then returns to the compressor (31). The secondary refrigerant repeats this circulation.
Particularly in the second and third solutions, the secondary refrigerant flowing out the evaporator (50) flows into the container (3a) of the receiver (34) from the second pipe (3c) and then flows out from the first pipe (3b). At the time, since the opening end of the first pipe (3b) is located at the lower position of the container (3a), the secondary refrigerant in liquid phase is easy to flow out. Further, since the shut-off valve (SVDL) of the pressure reduction passage (65) is slightly smaller in diameter than the passage, it provides resistance against refrigerant flow. As a result, a desired refrigerant circulating flow rate can be ensured.
On the other hand, the primary refrigerant in the primary side refrigerant circuit (20) discharges from the compressor (21) and then flows through the evaporation section of the refrigerant heat exchanger (11) to heat the secondary refrigerant in the secondary side refrigerant circuit (3A). Thereafter, the primary refrigerant having flowed through the refrigerant heat exchanger (11) flows through the receiver (25), evaporates in the condenser (22) and returns to the compressor (21). The primary refrigerant repeats this circulation.
Particularly in the first and third solutions, the primary refrigerant flowing out the refrigerant heat exchanger (11) flows into the container (2a) of the receiver (25) from the second pipe (2c) and then flows out from the first pipe (2b). At the time, since the opening of the first pipe (2b) is located at the upper position of the container (2a), the secondary refrigerant in liquid phase is hardly to flow out and the primary refrigerant in gas phase mainly flows out. As a result, it can be suppressed that the liquid refrigerant flows back to the compressor (21).
According to the first, third and fourth solutions, since the first pipe (2b) in the receiver (25) of the primary side refrigerant circuit (20) is arranged to be open at the inside upper position of the container (2a), a large amount of liquid refrigerant can be stored in the receiver (25). As a result, the primary refrigerant in liquid phase during the defrosting operation can be controlled at a suitable flow rate.
Specifically, when the outside air temperature is high, the evaporation capability of the condenser (22) is increased and in this case the first pipe (2b) mainly sucks the primary refrigerant in gas phase. Therefore, the liquid refrigerant does not flow back to the compressor (21). As a result, a wet operation can be consistently prevented thereby providing enhanced reliability.
Particularly, a wet operation can be prevented even for a long pipe with a large amount of refrigerant charged thereinto, and a wet operation can be prevented with reliability even if reduction in evaporation capability of the condenser (22) through fan control is insufficient.
Further, according to the second, third and fourth solutions, since the first pipe (3b) in the receiver (34) of the secondary side refrigerant circuit (3A) is arranged to be open at the inside lower position of the container (3a), the secondary refrigerant in liquid phase is easy to flow out. As result, the primary refrigerant in liquid phase during the defrosting operation can be controlled at a suitable flow rate.
Specifically, in the secondary side refrigerant circuit (3A), the amount of refrigerant charged therein is small and the capacity of the evaporator (50) is large, but the secondary refrigerant in liquid phase flowing into the receiver (34) returns to the compressor (31) with reliability. As a result, the refrigerant circulating flow rate during the defrosting operation can be consistently ensured thereby providing enhanced defrosting capability.
Particularly, since the shut-off valve (SVDL) of the pressure reduction passage (65) is slightly smaller in diameter than the passage, it provides resistance against refrigerant flow. This resistance allows the suction side pressure of the compressor (31) to be held at a predetermined value and therefore the secondary refrigerant in liquid phase evaporates in the refrigerant heat exchanger (11) and returns to the compressor (31) with reliability. As a result, a desired refrigerant circulating flow rate can be consistently ensured.